The present invention is related to a method for carrying out a handover between base stations of a mobile telecommunication network for a mobile terminal. The present invention also relates to a base station of a mobile telecommunication network provided for carrying out that method of handover. It also relates to computer programs in order for that method to be implemented.
Generally, in a telecommunication network, a mobile terminal is in communication with only one base station. However, it may happen that a second base station becomes more effective than the first base station for different reasons. In such a case, a handover process is realized consisting in switching the first base station that serves the mobile terminal, said the source base station, to the second base station, called the target base station.
For example, in traditional cellular systems (e.g. LTE), handover requires to setup a new radio channel between the mobile terminal and the target base station, and to release the old radio channel between the mobile terminal and the source base station.
Also, for example, in 3G cellular telecommunication networks, featuring soft handover, the RRC signaling is always established in the RNC and handovers can be viewed as a succession of Active Set Update (ASU) procedures, where the mobile terminal sees the number of its radio links vary over time, by successive extensions and reductions.
The present invention proposes a handover procedure in a telecommunication network wherein a mobile terminal is generally in communication with more than two base stations, one base station acting as a main base station and thus linked to the access gateway of the telecommunication network whereas the other base stations act as supplementary base stations and thus linked to the main base station. In such telecommunication network, a handover procedure is not so simple that the handover procedure of the above mentioned state of the art.
An example of such telecommunication network is now described in relation with FIG. 1.
FIG. 1 shows the elements of mobile telecommunication network involved in a telecommunication with a same mobile terminal 10. These elements are a base station 20, which here acts as a main base station in the network as it will be seen later on, a number N of base stations 301 to 30N (here, N=3), which act as supplementary base stations as it will be seen later on, and a gateway 40. N may either be predetermined or depend on transmission conditions over the channel between the mobile terminal 10 and the main base station 20 or a supplementary base station 30i. 
As to the mobile terminal 10, it includes a splitter/combiner 11, a main radio frequency transceiver 12 and N supplementary radio frequency transceivers 131 to 13N.
The main base station 20 includes a radio frequency transceiver 21 provided for communicating with the main RF transceiver 12 of the mobile terminal 10, a splitter/combiner 22, N inter base stations tunnel interfaces 231 to 23N and a gateway tunnel interface 24.
Each supplementary base station 30i includes an inter base stations tunnel interface 31i linked to a RF transceiver 32i. Each tunnel interface 31i is provided for being linked to a tunnel interface 23i in the main base station 20 in order to form a tunnel. The tunnel may be of the X2 type. As for each RF transceiver 32i, it is provided for communicating with a supplementary RF transceiver 13i in the mobile terminal 10.
The gateway 40 includes a tunnel interface 41 provided for being linked to the tunnel interface 24 of the main base station 20 and for receiving/transmitting the data flow to another terminal (not shown). The tunnel, created between the main base station 20 and the gateway 40 thanks to the tunnel interfaces 24 and 41, may be of the S1 type.
In the downlink, the other terminal (not shown) acts as a data source and feeds the gateway 40 and, particularly, the interface 41 of the gateway 40. The data flow is transmitted to the interface 24 of the main base station 20 and then to the splitter/combiner 22 where it is split into a main data sub-flow intended to be transmitted by the RF transceiver 21 to the mobile terminal 10 and N supplementary data sub-flows intended to be transmitted via the tunnel interfaces 231 to 23N of the main base station 20 and the tunnel interfaces 311 to 31N of the supplementary base stations 301 to 30N to the supplementary base stations 301 to 30N. Each supplementary data sub-flow is then transmitted to the mobile terminal 10 via the RF transceiver 32i of a supplementary base station 30i and is received by the RF transceivers 13i of the mobile terminal 10.
Note that each data sub-flow is constituted of data packets, each having a sequence number SN_DL assigned by the splitter 22. Whether the splitter 22 forks a data packet in the main data sub-flow or in the supplementary data sub-flow may for example depend on quality estimates reports sent by the mobile terminal 10 to the main base station 20 either directly or via the supplementary base stations 30 and/or may depend on quality estimates reports sent by the supplementary base station 30.
The combiner 11 of the mobile terminal 10 combines all the data sub-flows received by the main transceiver 12 and the supplementary transceivers 131 to 13N and delivers the recombined data flow to the output of the mobile terminal 10.
In the uplink, the mobile terminal 10 acts as a data source and feeds the splitter/combiner 11 of the mobile terminal 10. Here, the splitter 11 splits the data flow into a main data sub-flow intended to be transmitted to the RF transceiver 12 where it is sent to the main base station 20 and into N supplementary data sub-flows intended to be transmitted to the RF transceivers 131 to 13N where they are sent to the respective supplementary base stations 301 to 30N. In each supplementary base station 30i, the received supplementary data sub-flow is transmitted to the main base station 20 via the tunnel formed between the tunnel interface 311 of the supplementary base station 30i and the tunnel interface 23i of the main base station 20.
The main data sub-flow received by the RF transceiver 21 and the supplementary data sub-flows received by the respective tunnel interface 23i are combined in the splitter/combiner 22 of the main base station 20 and the resulting data flow is delivered to the gateway 40 by means of the tunnel interface 24 of the main base station 20 and the tunnel interface 41 of the gateway 40 and then to the other terminal (not shown).
Note that the splitter 10 marks each data packet sent by the mobile terminal 10 with a SN UL sequence number that is incremented each time a data packet of the data flow arrives in the splitter 10. Consecutive out-of-sequence data packets received by the combiner 22 are then recombined by the main base station 20 to build back the initial data flow fed initially to the splitter 10, composed of in-sequence consecutive data packets.
For example, at the output of a splitter/combiner, each split data sub-flow is different on each of the sub-flows so that each data sub-flow corresponds to a portion of the flow of data that differs from other portions of the flow of data to which correspond the other data sub-flows. In order for a splitter/combiner to determine which portion of the data it will send to a transceiver, it may receive from all the transceivers transmission indicator information, i.e. information about the status of the FIFO of the RF transceivers (full, expanding, steady, shrinking, empty), about the estimated radio link quality of the physical interface between two RF transceivers, about the achievable data rate or about the load indication over this physical interface, etc.
Note that base stations 20 and 301 to 30N are all identical base stations of the mobile telecommunication network. When considering all the base stations in communication with a same mobile terminal 10, only one of them acts as a main base station 20 then carrying out the above described means 21 to 24 whereas the other base stations act as supplementary base stations 30i then carrying out the above described means 31i and 32i. Which base station acts as a main base station whereas the others act as supplementary base stations depends on different parameters and conditions, for example radio frequency telecommunication conditions, which are not of importance for the present description.
Note also that only the base station 20 that acts as a main base station is linked to the gateway 40 whereas the other base stations are linked to the base station acting as a main base station.
Note also that the mobile telecommunication network contains more than one mobile terminal, and that one base station 20 acting as a main base station for a given mobile terminal can simultaneously act as a supplementary base station for another mobile terminal.
In such telecommunication network, it may happen that the transmission conditions over at least one of the radio links change. For example, it may result in a change of the status of the FIFO of a transceiver, or of the estimated radio link quality, of the data rate, etc. Following that transmission conditions change, the set of base stations involved during the transmission with a same mobile terminal has to be changed, for example, by adding new supplementary base stations or by removing existing supplementary base stations. The change of the set of base stations may also result in that the base station that has acted up to now as a main base station has now to act as a supplementary base station, then carrying out the above described means 31 and 32 while one of the base stations involved during the transmission with a same mobile terminal and acting up to now as a supplementary base station has now to act as a main base station, then carrying out the above described means 21 to 24. In this case, in the following, the old main base station will be said the “source base station” while the supplementary base station that will become the main base station will be said the “target base station”. All the modifications of the set of base stations are achieved during a procedure which is called a handover procedure.